No date has been set for a hearing between Coyne and Unite, but three sources told the Guardian the inquiries were stepping up and one was expected to happen in the first quarter of next year.

The hearing will be crucial for the future of McCluskey, one of Jeremy Corbyn’s closest allies and the leader of Britain’s biggest union, who has been seen as vital to the Labour leader over two years of challenges from some of his MPs.

McCluskey’s position was secured after he won 59,067 votes (45.4%) compared with Coyne’s 53,544 (41.5%) on a turnout of just over 12% – a closer result than expected.

The certification officer can order an election rerun if he finds against Unite on any of the 10 complaints submitted by Coyne.

However, Labour sources said this would be an unusually bold decision. A complaint against the election of Dave Prentis as the general secretary of Unison found a breach of the rules last year but not enough to have made a material difference to the result and so no re-run was ordered.

One senior Labour source, who is not connected to either side in the dispute, said supporters of McCluskey were increasingly worried as it would be difficult for him to run again if the finding went against him.

A Unite spokeswoman said: “Unite is confident that our rules and democracy have been respected through the election process and that any hearing will show this to be the case.”

A spokesman for the certification officer said it did not give a running commentary on complaints and no date had yet been set for a hearing.

Coyne and his solicitors will submit documents to the certification officer, claiming 10 different rule breaches. The complaints centre on alleged breaches including allowing McCluskey to use databases while stopping Coyne from doing the same during the campaign; union employees actively seeking to prevent Coyne raising the legitimate question as to whether Union resources were improperly used to assist with the purchase of a luxury flat; and repeated harassment of Coyne and his supporters by union employees.

McCluskey and his supporters are also accused of acting unlawfully in conducting a “robo-calling” exercise to try to persuade members to vote in the final period leading up to the close of the ballot. Unite has consistently denied this.